What happened in the US strikes?

US officials say hundreds of fighters aligned with Syrian President Bashar al-Assad attacked positions of the US-backed Syrian Democratic Forces (SDF) near the town of Khursham in the Deir al-Zour province.

They crossed the Euphrates River, shelling an SDF base, where American advisers were present.

The US responded with air and artillery strikes on 7 February, thwarting the attack and forcing the pro-Assad fighters to retreat, the US officials say.

What about the Russian mercenaries?

"If Russians are among those who were killed, this would mark the first time a US air strike has killed Russians in Syria," CBS News' David Martin reported.

More details about those killed in the strikes began to emerge in the following days.

On Tuesday, associates of two Russian fighters confirmed to the BBC that they were killed on 7 February.

The associates named the dead as Vladimir Loginov, a Cossack from Russia's western Kaliningrad region, and Kirill Ananyev, a radical nationalist from Moscow.

Some media reports suggest that dozens of Russian fighters could have been killed.

They are believed to have been employed by a private Russian company called Wagner. The firm has not commented on the reports.

How did the Kremlin react?

On Tuesday, Russian President Vladimir Putin's spokesman declined to provide any details.

"We have details of servicemen of the Russian armed forces, who are taking part in the Russian military operation in support of the Syrian army," Dmitry Peskov said.

"We don't have information about other Russians who might be in Syria. I advise you to contact the defence ministry."

And responding to a further BBC question, the spokesman said: " And, of course, you have to realise that many countries of the world have quite large numbers of our compatriots - of Russians - and, of course, it's difficult to have detailed data.